Jared Lee Loughner, the suspected gunman in a shooting rampage at a Safeway supermarket in Arizona that left six dead and 14 wounded, including Rep. Gabrielle Giffords, reportedly planned the shooting in advance.

Authorities executed a search warrant at Loughner's home and found a letter inside a safe that Rep. Gabrielle Giffords wrote to Loughner on Aug 30, 2007, thanking him for attending a "Congress on Your Corner" event, according to an FBI agent affidavit filed with a five-count criminal complaint.

"Also recovered in the safe was an envelope with handwriting on the envelope stating "I planned ahead" and "My assassination" and the name "Giffords," along with what appears to be LOUGHNER's signature," the criminal complaint reads.

Federal charges have been filed against Loughner, who fired the shots during a political event.

The criminal complaint includes an attempt to kill a member of Congress, two counts of murder in the first degree and two counts of attempted murder.

This comes as authorities announced that the man previously thought to be associated with the suspect has been cleared of any involvement in Rep. Gabrielle Giffords shooting.

Fox News has confirmed that the white man in his 40s or 50s who was photographed by a security camera arriving at the "Congress on Your Corner" event at the same time as the suspect was eliminated as a person of interest.

FBI Director Robert Mueller said earlier on Sunday that Loughner, 22, may face additional charges under the domestic terrorism statute.

"This was an attack not only against dedicated public servants, but against our fellow citizens," he said.

"As you know, Jared Lee Loughner was subdued by brave quick thinking individuals at the scene. He was taken into custody by sheriff deputies and is now in federal custody and formal charges are expected this afternoon," Mueller said.

“Given this tragedy, all logical precautions are in place to best ensure the safety of other public officials.”

"There is no information at this time to suggest any specific threat remains," he said.

"We will continue to dedicate all necessary resources to every level of this investigation."

Saturday's shooting left six people dead, including U.S. District Judge John M. Roll and 9-year-old Christina-Taylor Green. Fourteen people, including Rep. Giffords, were wounded in the attack.

In an interview with Fox News' Megyn Kelly, Pima County (Ariz.) Sheriff Clarence Dupnik said a woman, Patricia Maisch, waiting on line to meet Congresswoman Giffords lunged at the gunman's magazine as he was about to reload.

"This is one of the most heroic acts I've ever seen," Dupnik said.

Then, the local sheriff said, the shooter inserted another magazine with 31 bullets, but it didn't fire. Two men then grabbed the gun from him.

Maisch later told Fox News that she's no hero.

"The magazine was secured," Maisch said. "I’m not the hero. The two gentlemen who secured him to the ground are the heroes."

After being shot in the head, Giffords was able to communicate with doctors through simple commands, hospital officials said at an earlier press conference on Sunday. The Arizona Democrat remained in critical condition following the mass casualty shooting Saturday that prompted Mueller to take lead of the investigation.

"What was most concerning to us with this injury is the "bullet crossing from one hemisphere to the other," Dr. Michael Lemole, chief of neurosurgery at University of Arizona Hospital, said during a press conference Sunday. That was not the case, in this instance, he said.

"And because of this, Congresswoman Giffords was able to communicate with us this morning through simple commands."

He gave an example of a simple command as "please squeeze two fingers," adding that we take such commands for granted, but "they imply a very high level of functioning in the brain."

"I am cautiously optimistic," Lemole said.

Lemole said the gunshot went through the left side of her head and that surgeons worked to reduce pressure from swelling in the brain by removing bone fragments.

Dr. Peter Rhee, head of trauma, critical care and emergency surgery, said, "Overall this is about as good as good can get" with a bullet wound to the head.

In all, Rhee said doctors performed six surgeries on the patients brought to the hospital.

"Those surgeries included a variety of things in a trauma-combat type of scenario. It was a mini mass causality in some senses," said Rhee, an Afghan and Iraq war veteran. "We were doing things from the chest to the abdomen, also vascular, cardio-thoracic, also orthopedic in nature as well. As of this morning we're happy to state that (there is) only one patient that remains in critical condition."

A federal law enforcement source told Fox News that it is the first time in their memory that the FBI director has been dispatched to a crime scene by the president.

The source said it's believed that President Obama wants the director to get first-hand accounts from commanders and to be, in effect, the president's eyes and ears on the ground.

Police said Giffords was the target of the shooting rampage at the town hall-style event Saturday in Tucson.

The federal law enforcement source cautioned that the investigation is not even a day old, but said investigators' working theory is that Loughner acted alone and that there is no evidence so far that suggests the shooting spree was a conspiracy involving one or more individuals.

Given the death of federal judge John Roll, the law enforcement source told Fox News that the U.S. attorney has recused all federal judges in the district from signing arrest or search warrants because of a potential conflict of interest in the case. The searches and warrants executed so far are described as state warrants.

Investigators have pulled all surveillance video where the suspect appears and they are actively tracking anyone who appears with Loughner, the source said. These additional individuals are not described for the time being as suspects, but rather as potential witnesses.

Details remain murky. However, the shooting occurred in a shopping center with chain stores, so video cameras were likely in the vicinity and authorities may have more footage to help them with the investigation.

A federal law enforcement source characterized Loughner as refusing to talk to agents and said he had invoked his right to counsel.

The source added that the processing of the crime scene at the Safeway in Tucson is nearly complete. Investigators also have Loughner's computers and cell phones and are tracking back his contacts and Internet traffic in the months and days leading up to the attack.

The motive in the shooting remains unclear. Dupnik described the shooter as "unstable" with a troubled past.

Although Loughner is reportedly not talking with police, the investigation continues and some elements are becoming clearer.